As we learn in the books:
“The only people who can see thestrals,” [Luna] said, “are people who have seen death.”
— Order of the Phoenix, chapter 21 (The Eye of the Snake)
Muggles see, and are touched by, death just as often as wizards would be. Now, the first step to seeing a thestral is being near one. But during the course of the Harry Potter books thestrals were flying over London several times.
In fact, there are two thestrals walking around Privet Drive prior to the Battle of the Seven Potters. Fleur Delacour with Bill Weasley, and Hermione Granger with Kingsley Shacklebolt both left on thestrals and they were all inside for several minutes while the steed were out front. Given that Little Whinging isn't a wizard enclave it is certainly possible someone saw them.
So given that, is it possible for a muggle to see a thestral if they have “seen death”?